Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). In flex-fuel vehicles, a flexible multiple-fuel engine can operate with fuel of varying composition by delivering different proportions of the variable fuel to different injectors. For example, direct injection of an ethanol fuel may be used to take advantage of the increased charge cooling effects of the alcohol fuel's higher heat of vaporization and increased octane. This helps to address knock limitations, especially under boosted conditions.
Direct injected engines, however, also generate more particulate matter emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. Similarly, the injected fuel may encounter less turbulence when flowing through the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions.
One approach for reducing particular matter emissions generated by direct injection of fuel is shown by Bidner et al. in US2011/0162620. Therein, an amount of fuel injected into the cylinder, between the direct injector and the port fuel injector, is adjusted based on the amount of particulate matter (PM) produced by the engine. For example, as a soot load increases, a fuel injection amount from the direct injector is decreased while a fuel injection amount from the port injector is correspondingly increased.
However, the inventors herein have identified potential issues with such an approach. During selected engine operating conditions, even with the shift towards more direct injection, particular matter emissions may not be sufficiently reduced to meet the mandated low PM emission standards. For example, the direct injection may be performed too late such that by the time the direct injection occurs, the emitted PMs are higher than emission standards. The problem may be exacerbated in flex-fuel vehicles due to the varying nature of the fuel. For example, based on when tank refueling occurs (e.g., when the tank is empty or partially full), and what type of fuel the fuel tank is refilled with (e.g., whether the tank is refilled with gasoline, E10, E50 or E85), the composition in the fuel tank may change. This affects the amount of soot generated by subsequent direct injection of the fuel. Further still, during conditions when an alcohol fuel is used, the charge cooling effect of the alcohol fuel on the intake system can result in lower air-charge temperatures at cold-start conditions. This can degrade combustion stability and increase potential for engine misfire. Overall, fuel economy and cold-start exhaust emissions may be degraded if the system is not optimized for the operating conditions and fuel types present.
The above issues may be at least partly addressed by a method for an engine comprising: during a first combustion event since engine start, port injecting a first amount of fuel during a closed intake valve event, and direct injecting a second amount of fuel over multiple injections of the first combustion event, the first and second amounts based on an alcohol content of the injected fuel. In this way, benefits from alcohol fuels as well as from a fuel injection split between port injection of a first fuel and (single or multiple) direct injection(s) of a second fuel can be synergized.
In one example, during an engine start, an engine control system may inject fuel into a cylinder, on the first cylinder combustion event, as a first port injection delivered during a closed intake valve event (e.g., during an exhaust stroke), a second direct injection delivered during a compression stroke, and a third direct injection during an intake stroke. This may constitute a first injection profile. The same injection profile may be continued during cranking for a number of combustion events, based on the alcohol content of the fuel. For example, as the alcohol content of the injected fuel increases, the first injection profile may be continued for a larger/smaller number of combustion events. Further, the number of threshold cylinder event number (e.g., up to cylinder event number 24). By injecting a portion of the fuel as a port injection and a remaining portion of the fuel as a direct injection, an exhaust catalyst temperature can be rapidly increased to a light-off temperature, improving engine performance at engine cold-starts. By also splitting the direct injection so that some of the direct injected fuel is injected during the compression stroke and the remaining part of the direct injected fuel is injected during the intake stroke, the catalyst light-off temperature can be attained without raising exhaust particulate matter (PM) emissions and degrading engine combustion stability. At the same time fuel economy is improved. After a target cylinder combustion event number is reached, the injection profile may be transitioned to a second injection profile that is configured for idle engine speed control. The second injection profile may include, for example, only port injection of fuel, only direct injection of fuel, and/or a split ratio that is different from the split ratio of the first injection profile, with a higher percentage of direct injected fuel. In still further embodiments, the injection profile during an engine cold start may vary based on the engine temperature at the cold start (e.g., based on whether it is a regular temperature engine cold start or a very cold temperature engine cold start).
In this way, by using a split injection profile that splits a fuel injection between a port injection and multiple direct injections, an activation time for an exhaust catalyst can be reduced while producing lower gaseous and particulate matter emissions. At the same time, a higher amount of spark retard can be tolerated without affecting combustion stability. As such, this allows fuel injection to be optimized so as to enable the benefits of a fuel injection split between a port injection and direct injection to be synergized with the benefits of multiple direct injections. Overall, engine performance is improved, exhaust emissions are improved and further, fuel economy is also improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.